Various optical displacement encoders are known that use a readhead having an optical arrangement that images a scale pattern to a photodetector arrangement in the readhead. The image of the scale pattern displaces in tandem with the scale member, and the movement or position of the displaced scale pattern image is detected with a photodetector arrangement. Conventional imaging, self-imaging (also called Talbot imaging), and/or shadow imaging may be used to provide the scale pattern image in various configurations.
Optical encoders may utilize incremental or absolute position scale structures. An incremental position scale structure allows the displacement of a readhead relative to a scale to be determined by accumulating incremental units of displacement, starting from an initial point along the scale. Such encoders are suitable for certain applications, particularly those where line power is available. However, in low power consumption applications (e.g., battery powered gauges, and the like), it is more desirable to use absolute position scale structures. Absolute position scale structures provide a unique output signal, or combination of signals, at each position along a scale. They do not require continuous accumulation of incremental displacements in order to identify a position. Thus, absolute position scale structures allow various power conservation schemes. A variety of absolute position encoders are known, using various optical, capacitive or inductive sensing technologies. U.S. Pat. Nos. 3,882,482; 5,965,879; 5,279,044; 5,886,519; 5,237,391; 5,442,166; 4,964,727; 4,414,754; 4,109,389; 5,773,820; and 5,010,655, disclose various encoder configurations and/or signal processing techniques relevant to absolute position encoders, and are hereby incorporated herein by reference in their entirety.
One issue with regard to the design of optical encoders is that users generally prefer that the readheads and scales of the encoders be as compact as possible. A compact encoder is more convenient to install in a variety of applications. However, absolute optical encoders generally require a plurality of scale tracks, which tends to demand a relatively broader scale member, illumination system, and photodetector arrangement. One solution has been to minimize the width of absolute scale tracks. However, minimizing the width of scale tracks, the illumination system, and photodetector arrangement is generally detrimental to the signal to noise ratio(s) of an absolute encoder, reducing its potential accuracy. Furthermore, narrower scale tracks are relatively more sensitive to a given amount of contamination, misalignment, and other variations that may be expected in industrial environments, and are therefore less robust and stable in operation. Thus, in particular with regard to absolute optical encoders, it would be desirable to maintain relatively larger scale track widths, in order to maintain accuracy as robustly as possible, while otherwise maintaining as compact an optical encoder configuration as possible. However, the prior art fails to teach configurations which provide certain combinations of robustness, compact size, range-to-resolution ratio, and cost desired by users of encoders. Improved configurations of encoders that provide such combinations would be desirable.